Heretofore the American National Standard Standards Institute (ANSI) has published a draft standard, document X3.186-199x, entitled "FDDI Hybrid Ring Control" for a family of optical fiber ring network protocols using ring topology known as the fiber distributed data interface, FDDI. The draft protocol postulates a hybrid ring in which time division multiplexed, circuit-switched services may be integrated with variable rate packet switched services. The isochronous data required for the circuit-switched services as well as the packet data are proposed to be carried in special, fixed-length, fixed-duration frames called cycles having a repetitive frame length of 125 microseconds (.mu.s). The cycle is shown in FIG. 1 hereof and briefly described herein.
The FDDI proposal further contemplates that a hybrid multiplexer at a master station, called a cycle master, would include a "latency adjustment" buffer (LAB) to ensure that isochronous data will take an integral multiple of 125 .mu.s to travel around the ring. However, the efficient management of isochronous data and packet data requires that conflicting criteria be observed since isochronous data occurs at precise time intervals in fixed amounts, while packet data has an arrival process that is not fixed and may be modelled as exponential or gaussian. In a practical system, isochronous data must be transmitted in exactly the same relationship to the beginning of a cycle as that in which it was received, while packet data must be transmitted with minimum delay between the receipt of a packet symbol and its retransmission, even though this may entail the crossing of cycle boundaries, without regard to what is happening with respect to isochronous data. Accordingly, the realization of a practical latency adjustment buffer has proven to be somewhat elusive. Moreover, in a practical network, sections may be added to accommodate network growth and at other times sections may be deleted or bypassed. Accommodating such changes without shutting down the network presents a major challenge.